Incandescent lamps such as tungsten filament lamps or halogen lamps are widely used as sources of artificial light. In the early stage, incandescent lamps are used for simply providing a bright place. With diversified living attitudes, incandescent lamps having difference brightness are developed. For adjusting brightness of respective incandescent lamp, a brightness-adjustable circuit is used to drive the incandescent lamp and control the brightness of the incandescent lamp.
FIG. 1 is a schematic circuit diagram illustrating a brightness-adjustable circuit for a conventional incandescent lamp. As shown in FIG. 1, the brightness-adjustable circuit 1 includes a switch element 11 and a triggering circuit 12. The switch element 11 is for example a solid semiconductor component such as a silicon-controlled rectifier (SCR) or a TRIode for Alternating Current (TRAIC) component. Take a TRAIC component as the switch element 11 for example. The control terminal G is the gate of the switch element 11. The first terminal T1 and the control terminal G of the switch element 11 are coupled to the incandescent lamp 13 and the triggering circuit 12, respectively. The second terminal T2 of the switch element 11 can receive the electric energy from the input voltage Vin. The triggering circuit 12 can control the on phase or on duration of the switch element 11, thereby controlling the electricity to be transmitted to the incandescent lamp 13.
Please refer to FIG. 1 again. The triggering circuit 12 includes a resistor R, a variable resistor Rvar, a capacitor C and a bidirectional diode thyristor D. The resistor R, the variable resistor Rvar and the capacitor C are connected in serried with each other to form a charging loop. Both ends of these serially-connected components are coupled to the second terminal T2 of the switch element 11 and the incandescent lamp 13, respectively. An end of the bidirectional diode thyristor D is coupled to the control terminal G of the switch element 11. The other end of the bidirectional diode thyristor D is coupled to the capacitor C. Through the charging loop which is defined by the resistor R, the variable resistor Rvar and the capacitor C, the input voltage Vin, can charge the capacitor C. Until the capacitor C is charged to the turn-on voltage of the bidirectional diode thyristor D, the bidirectional diode thyristor D is conducted and thus a triggering signal is transmitted to the control terminal G of the switch element 11. In response to the triggering signal, the switch element 11 is conducted. That is, the on phase or on duration of the switch element 11 can be controlled by adjusting the resistance of the resistor R, thereby controlling the electricity to be transmitted to the incandescent lamp 13 and adjusting the brightness of the incandescent lamp 13.
In recent years, light emitting diodes (LEDs) capable of emitting light with high brightness and high illuminating efficiency have been developed. In comparison with a common incandescent light, a LED has lower power consumption, long service life, and quick response speed. With the maturity of the LED technology, LEDs will replace all conventional lighting devices. Until now, LEDs are widely used in many aspects of daily lives, such as automobile lighting devices, handheld lighting devices, backlight sources for LCD panels, traffic lights, indicator board displays, and the like.
The brightness-adjustable circuit is only applicable to the incandescent lamp with the pure resistive property. On the other hand, the conventional LED driving circuit is operated according to the non-pure resistive property of the LED. Generally, there is often a phase difference between the input current and the input voltage at the input side of the conventional LED driving circuit and the waveforms of the input current and the input voltage are very distinguished. If the LED driving circuit and the brightness-adjustable circuit are simultaneously used, the LED possibly flashes or the LED driving circuit or the brightness-adjustable circuit is readily burnt out because the LED driving circuit can only receive power signals with constant on phase or on duration. Moreover, the conventional LED driving circuit fails to receive the power signals which are subject to brightness regulation and have varied on phase or on duration. In other words, the conventional LED driving circuit fails to cooperate with the brightness-adjustable circuit.
There is a need of providing a brightness-adjustable LED driving circuit to obviate the drawbacks encountered from the prior art.